Interstitial pulmonary fibrosis is a lethal, chronic response to lung injury. Although numerous agents can elicit interstitial fibrosis in the lung, no eliciting factors can be identified in many cases. The biochemical mechanisms leading to diffuse interstitial pulmonary fibrosis are poorly characterized. Increasing evidence indicate that reactive oxygen species and proteases are involved in the pathogenesis of this disease. The antioxidant enzyme extracellular superoxide dismutase (EC-SOD) is highly expressed in the lung and is the major antioxidant enzyme in the extracellular matrix. EC-SOD has a heparin-binding domain that regulates its biodistribution in the extracellular matrix. This domain is sensitive to proteolysis, and proteolytic removal of the heparin-binding domain may be a primary mechanism to regulate EC-SOD levels in extracellular spaces. We found that this enzyme protects against pulmonary fibrosis and that its distribution is significantly changed as a result of proteolysis in a bleomycin model of pulmonary fibrosis. Latent metalloproteases, enzymes that degrade components of the extracellular matrix including collagen, can be activated by oxidants such as superoxide and have been implicated in the pathogenesis of pulmonary fibrosis. We have found that these proteases can also cut EC-SOD's heparin-binding domain. Thus, oxidants can increase the protease/antiprotease balance in the lung that can then lead to proteolytic clearance of EC-SOD from the lung resulting in an oxidant/antioxidant imbalance. This proposal will investigate the synergistic effects of oxidants and proteases in the pathogenesis of pulmonary fibrosis.